1. Field of the Invention
The present invention is related to a programmable attenuator.
2. Background Art
Communication systems allow for communication over large distances by using sophisticated transponders in a satellite. Transponders receive incoming communications over a band of frequencies, the uplink, and retransmit the communications over another band of frequencies, the downlink, at the same time.
The uplink originates from a point on surface of the Earth, usually a ground station, to the transponder. A receiver in the transponder receives the incoming communication during the uplink. The transponder sends the downlink to a point or region on the surface of the Earth, usually to one or more ground stations or receivers. Separate receivers located on the surface of the Earth receive the retransmission of the communication during the downlink.
The receiver in the transponder, as well as the receivers located on the surface of the Earth, may be implemented as either a heterodyne receiver or a homodyne receiver, also called a direct-conversion receiver. A receiver constructed using a heterodyne architecture down-convert the received communication into a much lower frequency, also called an intermediate frequency (IF). The down-conversion process in the heterodyne architecture generates images of the communication. The most common approach to suppressing the images is through the use of an image reject front-end filter. On the other hand, direct-conversion receivers down-convert the received communication to baseband thereby eliminating the need for IF stages and the image-rejection requirement of front-end filters. As a result, direct-conversion can significantly improve the on-chip integration of a receiver.
Implementing a direct-conversion tuner for satellite broadcasting is challenging due to wide input signal bandwidths (e.g., ranging from 950˜2150 MHz) and low received power levels (e.g., ranging from −25˜−70 dBm). In addition, due to the many broadcasting channels and satellites, Digital Broadcasting System (DBS) receivers must contend with large numbers of unwanted channel interferers. Consequently, the radio frequency (RF) and the baseband attenuators of a DBS tuner should exhibit good linearity over a wide tuning range to enable the DBS tuner to achieve high channel-selectivity.